Beavers as ecosystem restoration agents across human land-use gradients and habitat boundaries

Biodiversity declines due to human activities are ongoing, with freshwater habitats especially degraded and fragmented. Restoration actions for these habitats need to be cheap and large-scale to be effective, but often empirical testing of restoration methods is missing. Ecosystem engineers have been suggested as an option to restore freshwater ecosystems. The recolonization of beavers (Castor fiber in Eurasia, C. canadensis in North America) offers the chance to leverage the actions of such an ecosystem engineer. Beavers modify ecosystems across aquatic and terrestrial boundaries, which can benefit the ecosystem functionality, biodiversity and community composition of plant and animal species of the whole ecosystem. However, so far, most studies on beaver effects took place in pristine systems, not reflective of the situation in much of the former range. Additionally, while it is known that beavers benefit biodiversity and ecosystem functionality, the pathways leading to these changes are poorly understood. To address these knowledge gaps, we recorded biodiversity and abundance data, ecosystem functions and characteristics across aquatic-terrestrial boundaries in 16 stream systems with a land-use gradient in Switzerland in a beaver-engineered area with a beaver dam and a control site without beavers along the same stream. We found that even in highly human-modified landscapes, beaver-engineered biodiversity benefits persist, although with smaller positive effects for some species. Surprisingly, in more human-dominated areas, beaver-engineering led to a higher species turnover, underscoring the ability of beavers to effectively create new habitats. By studying the bats in beaver systems, we could show that both food availability and habitat heterogeneity drive these changes in biodiversity. These results highlight the potential of beavers as an ecosystem restoration agent even in degraded, high-human land-use systems. Our research shows that beavers have enormous potential for restoration because they restore significant parts of the biodiversity of freshwater ecosystems. For restoration and beaver management, we could show the value of integrating and tolerating beaver activity, also in highly human-altered landscapes. For ecology, we could disentangle the mechanisms leading to biodiversity change in the presence of an ecosystem engineer.